1. Field of the Invention
The invention in general relates to liquid status detectors, such as those that detect hydrocarbon liquid leaking from underground storage tanks, and in particular a detector that combines both internal tank inventory sensors and external leak sensors in a single system.
2. Description of the Prior Art
U.S. Pat. No. 4,221,125, on an invention of John N. Oliver and Louis M. Sandler, U.S. Pat. No. 4,646,069 issued to Raymond J. Andrejasich et al, U.S. Pat. No. 4,660,026 issued to Brian L. Chandler and U.S. Pat. No. 4,116,045 on an invention of Bronson M. Potter, are exemplary of systems for detecting the presence of leaking liquid. Typically such systems include leakage probes that are buried in the vicinity of hydrocarbon storage tanks, placed between the walls of double-walled hydrocarbon storage tanks, or otherwise placed externally of storage tanks to detect liquid leaking from the tanks. The probes are generally connected via wires to a central controller, which may be located, for example, in a service station office, and which monitors the probe status. In addition, a wide variety of systems have been known for many years for measuring the liquid level within tanks for the purposes of gauging the amount of liquid in the tank and or for detecting the leakage of liquid from the tank. See for example U.S. Pat. No. 2,775,748 issued to R. L. Rod, et al, U.S. Pat. No. 3,017,771 issued to F. R. Bonhomme, U.S. Pat. No. 4,571,987 issued to J. A. Horner, U.S. Pat. No. 4,604,893 issued to F. J. Senese, et al; U.S. Pat. No. 4,637,254 issued to J. F. Dyben, et al; U.S. Pat. No. 4,646,560 issued to J. W. Maresca, Jr., et al, and U.S. Pat. No. 4,646,569 issued to H. F. Cosser and European Patent Application No. 1,106,677 on a disclosure of J. S. Haynes. The foregoing references reflect just a small portion of the available liquid level measuring devices. Despite the extensive research and development that has gone into liquid level gauging, the preferred method of taking inventory in underground storage tanks remains the dipstick. The present invention has for the first time combined external leak detection sensors with internal level gauging sensors in a unique system which results in important advantages, particularly for underground gasoline storage tanks, which system makes electronic inventory control practical.